The present invention relates to brush seals for rotary machines such as steam and gas turbines and particularly relates to brush seals and labyrinth-brush seal combinations for use with rotary machines.
Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines consisting of multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components, and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent on the ability of the seals to prevent leakage, e.g., between the rotor and stator. Turbine designs are conventionally classified as either impulse, with the majority of the pressure drop occurring across fixed nozzles, or reaction, with the pressure drop more evenly distributed between the rotating and stationary vanes. Both designs employ rigid tooth, i.e., labyrinth, seals to control leakage. Traditionally, rigid labyrinth seals of either a hi-lo or straight shaft design are used. These types of seals are employed at virtually all turbine locations where leakage between rotating and stationary components must be controlled. This includes interstage shaft seals, rotor end seals, and bucket (or blade) tip seals. Steam turbines of both impulse and reaction designs typically employ rigid, sharp teeth for rotor/stator sealing. While labyrinth seals have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a xe2x80x9cmushroomxe2x80x9d profile and opening the seal clearance.
Another type of seal used in many environments, including rotary machines, is a brush seal. Brush seals are generally less prone to leakage than labyrinth seals. A brush seal can also accommodate relative radial movement between fixed and rotational components, for example, between a rotor and a stator, because of the flexure of the seal bristles. Brush seals also generally conform better to surface non-uniformities. The result of using brush seals is better sustained rotary machine performance than is generally possible with labyrinth seals.
In accordance with a preferred embodiment of the present invention, there is provided brush seal segments in the environment of a rotary machine such as a turbine. Brush seals per se have general applicability to rotary machines and can be used in lieu of labyrinth seals. Brush seals are advantageous in that context and provide improved sealing, while occupying considerably less axial space as compared with conventional labyrinth seals. As a result, more compact rotary machine, e.g., turbine, designs can be realized. Alternatively, by employing brush seals, the span that would normally be occupied by labyrinth teeth can be used to allow additional turbine stages, resulting in increased turbine efficiency. As a further advantage, application of brush seals at end packing locations can reduce leakage to the point that the need for a gland sealing/exhauster system, for example, in a steam turbine, is eliminated. At rotor end seals, it is also possible to use brush seals in conjunction with face seals. Further, in certain steam rotary machine applications, some leakage is desirable for cooling of components such as rotors. At these locations, brush seals can be used in conjunction with orifices or other flow bypass mechanisms to ensure that the proper amount of leakage is obtained.
A typical brush seal for use in the present invention comprises a bristle pack, i.e., bristles sandwiched between two metallic plates. The bristles are generally alloy steel wires, drawn to a diameter of 0.002-0.006 inches, although the exact diameter depends on the specific seal application. Larger wire diameters are used for seals exposed to a high pressure differential between the upstream and downstream sides. The backing (downstream) plate, or in the present invention a labyrinth tooth, prevents the bristles from deflecting axially under pressure load. As a result, fence height (h) is a critical design variable. Fence height is the distance the bristles extend freely from their support, i.e., the distal end of the support plate or contact points between the bristles and the labyrinth tooth, to their free ends, which typically are in engagement with the rotating component. For a steam turbine application, where the expected maximum radial rotor deflection is approximately 0.040 inches, the fence height must therefore be a minimum of 0.040 inches. Fence heights vary significantly, particularly in gas turbines, depending on the seal location, from 0.030 for bearing seals, to 0.120 for high pressure packing seals to 0.300 for turbine interstage seals.
During shaft radial excursions, the bristles must be able to temporarily deflect without buckling. In order to accommodate these excursions, the bristles are not oriented in a perfectly radial direction, but are instead canted at some angle. Typically, this angle is between 30 and 60 degrees. Increased angles are used to allow for increased radial shaft excursions. If the bristles were straight in a radial direction, the rotor would interfere with the bristles and the bristles would act as columns rather than deflect as beams. This would result in increased wear and not accommodate radial excursions of the shaft.
The bristles of brush seals are typically mounted between a pair of plates or arcuate segments, with the bristles and plates being welded to one another at the ends of the bristles remote from the tips engaging with the opposite component of the seal. Conventionally, the brush seal is cut into a number of segments, typically four, with the cuts at the ends of each of the segments oriented at the same angle as the xe2x80x9ccantxe2x80x9d angle of the bristles. That is, with the angle of the bristles typically being on the order of 45xc2x0 relative to radii of the arcuate segments, the ends of each of the segments are likewise cut at the same angle and therefore parallel the linear extent of the bristles as they project from the segment at that segment end. As a result, the bristles lie at an angle affording the capability of accommodating radial excursions of the rotating component. By cutting the segments of the seal at the angles of the bristles, the bristles may be secured at that angle in the segments without loss of bristles, resulting in an assembled seal with a full 360xc2x0 of bristles for maximum sealing effectiveness.
In certain applications, however, for example conventional labyrinth seals, the labyrinth seal segments are generally cut in a radial direction for ease of manufacturing and assembly. Further, when retrofitting brush seals to existing labyrinth seals or supplying brush seals as original equipment in combination with labyrinth seals, it has been found desirable according to the present invention to cut the brush seal segment ends in a radial direction rather than in the direction of the xe2x80x9ccantxe2x80x9d of the bristles. This results in making surfaces at the segment end interfaces that are perfectly straight along radii of the segments with no interlocking pieces and no projections that can be damaged during assembly. However, cutting brush seal segments at the ends of the segments inconsistent with the bristle orientation angle, i.e., the cant angle, results in the loss of an area of the bristles. For example, where a brush seal is employed on a fixed component for sealing about a rotating shaft, the bristles of the brush seal are attached to the seal segments along the outer diameter and project radially inwardly at the cant angle. With the ends of the seal segments cut along radii, there are areas at the juncture of the seal segments where no xe2x80x9ccantedxe2x80x9d bristles are present. That is, with the bristles canted, for example, at a 45xc2x0 angle, and the end segments lying along radii, there is a triangular area on one end of one segment in which bristles do not project. However, in accordance with the present invention, the bristles are backed by a support plate in the form of a labyrinth tooth profile. Thus, the labyrinth tooth extends the full 360xc2x0 about the seal, serving as a back-up seal in the event of bristle damage and serving as the primary seal in those areas where the bristles are absent by design.
This particular design is useful as a retrofit seal with conventional labyrinth seals or may be used as a stand-alone seal. It is significant in that by cutting the end surfaces of the segments in a radial direction, the individual segments are free to move radially independently of one another. This is particularly important when the brush seals are retrofit to labyrinth seal segments that are spring-backed or held in place by pressure forces and therefore free to move radially during severe rotor/seal interferences.
In a preferred embodiment according to the present invention, there is provided an annular brush seal for disposition between a rotatable component having an axis of rotation and another component about the axis, comprising a plurality of arcuate segments for forming an annulus about and lying in a plane normal to the axis and between the components, each segment including an arcuate array of discrete, generally linearly extending bristles secured to the segment and extending therefrom at angles offset from radii of the segments, each of the segments having opposite ends along radii thereof for substantial registration with ends of adjoining segments such that, upon assembly of the segments into an annulus, the opposed ends of each pair of adjoining segments lie substantially along a radius of the annulus with the bristles extending at angles offset from radii of the segments.
In a further preferred embodiment according to the present invention, there is provided a rotary machine comprising a rotatable component, a component fixed against rotation, the components lying about a common axis, a brush seal about the axis and between the components, the brush seal including a plurality of arcuate segments forming an annular ring about the axis, each segment having opposite ends having end faces along radii of the axis for substantial registration with end faces of adjoining segments, such that the opposed end faces of each pair of adjoining segments lie substantially along a radius of the ring about the axis, with each segment having an arcuate array of discrete, generally linearly extending bristles secured thereto and extending from the segment at angles offset from radii of the segments at the circumferential location of the bristles.
Accordingly, it is a primary object of the present invention to provide a novel and improved brush seal and combination labyrinth-brush seal for sealing between components rotatable relative to one another and particularly for retrofitting brush seals into labyrinth seals segmented for movement radially by spring or pressure forces.